PCI: fix AER capability check

The 'use pci_find_ext_capability everywhere' cleanup brought a new bug,
which makes the AER stop working.  Fix it by actually using find_ext_cap
instead of just find_cap.  Drop the unused config space size define while
we're at it.

Signed-off-by: Yu Zhao <yu.zhao@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>